Using transformer at half its rated voltage (both pri and sec)
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  1. #1
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    Default Using transformer at half its rated voltage (both pri and sec)

    In general, is it a problem to use transformers at voltages lower than their nameplate, as long as you expect an output that is proportionally less, as well?

    My current application is to hope to use the below linked square d transformer (460/575 pri, 230/115 sec), using the 460 to 230 terminals (2:1) to transform 230 down to 115.

    3SQ58110 - TRFMR DRY 1PH 3KVA 480/575V-120/240V | Schneider Electric

    Conversations about transformers (flux and saturation) sound like Back to the Future to me, lol.

    Thanks in advance!

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    Not at all. The wattage will go down with the decrease in voltage.

    Tom

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    What you wish to do will work.

    The only downside I see is that (and I think this is correct) is that at full load if there was a 5% voltage drop at the higher voltage there would be a 10% drop at the lower voltage. In other words, the voltage drop in volts would be the same at full current in both the normal and reduced voltage configurations, but that would be a larger percentage of the lower voltage.

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    The voltage drop is due to "impedance", whch is resistance and inductance. Those do not change with voltage, since you are not changing connections, you are using a 460V unit on 230V..

    The transformer will permit the same current, and no more, at the lower voltage as at the higher voltage. Any more will cause overheating at any voltage. So, the rating in VA will be half at half voltage.

    Yes, the impedance will produce more voltage drop as a PERCENTAGE at the lower voltage. So a transformer that was a 3% unit at 460V would be more like a 6% unit at 230V input. The available fault current will be reduces as well, for the same reason.

    As long as you limit the current to the same as was allowed at 460V, it will generally work the same, with about double the percent voltage drop under load.

    The :iron loss" will be considerably less also, but overall, that will not make much difference.

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    Quote Originally Posted by JST View Post
    The :iron loss" will be considerably less also, but overall, that will not make much difference.
    True, with the caveat that it depends how deep into saturation the original design went. With typical core iron, saturation is over a considerable range. Running at half voltage will probably mean the core peaks at about 7,000 Gauss, where losses are low. You will still have magnetization losses, so the actual losses are hard to predict. One time I was working on an electronic balance for measuring specific gravities of liquids and had trouble with temperature drift. The light bulb was run by a little transformer from Ogden Coil & Transformer. When I asked them if they could reduce the temperature rise of their transformer, they increased the core stack, in effect making a transformer with a 150 or 160 volt primary that was run at 120 volts. They ran quite noticably cooler.

    Bill

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    Thanks for the help. So, in keeping an eye not to go over the design amperage, do I need to watch the pri or sec limit, or don't violate either?

    In other words, the nameplate allows for 460 or 575 to 230 or 110. So in the highest amperage use case the nameplate allows for, you'd be converting 460 to 110. So 3kva at 460v in is 6.5A and the 110v out is 27A. In my use case I want to take 230 to 110, so if I maintain the 110v limit of 27A, that's 3kva. But, if I have to respect the 6.5A nameplate limit of the primary, then 6.5A at 230v is only 1.5kva.

    So do I need to stick to the 6.5A primary limit, derating the transformer by half, or am I ok as long as I don't violate the 27A secondary limit, giving me the full 3kva?

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    As JST stated the VA capability will be halved. VA is shorthand for the implied math of Volts times Amps. The VA of the transformer will be half of the rating which in this case is half of 3 KVA resulting in 1.5 KVA.

    Ignoring heating of the transfomer, which is a small part of the power consumed by the transformer, at full load the input VA will equal the output VA. By using VA as the rating the transformer company does not have to state a current for each of the possible combinations.

    Do not load the transformer past 1.5 KVA if using it on half voltage.

    Bill

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    what is the reason you need to do this?

    You can use the secondary alone as a 240/120v auto transformer, in which case you can get 3kva worth of 110 volts out of it. (you will also get 480 out of the primary)

    If you have multiple 120 volt loads and if they don't need to be on the same circuit then the transformer only has to supply the difference if you wire the loads properly.

    If you have to use the transformer as an isolation transformer at half voltage then you will only get 1.5kva out of it (you can also get 60 volts out of it in such a case, not sure that would be of any use to you, but if you ground the center tap of the secondary you would have 60 volts line to ground on both lines. this could be useful if safety is a high priority.

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    Quote Originally Posted by johansen View Post
    what is the reason you need to do this?

    You can use the secondary alone as a 240/120v auto transformer, in which case you can get 3kva worth of 110 volts out of it. (you will also get 480 out of the primary)

    If you have multiple 120 volt loads and if they don't need to be on the same circuit then the transformer only has to supply the difference if you wire the loads properly.

    If you have to use the transformer as an isolation transformer at half voltage then you will only get 1.5kva out of it (you can also get 60 volts out of it in such a case, not sure that would be of any use to you, but if you ground the center tap of the secondary you would have 60 volts line to ground on both lines. this could be useful if safety is a high priority.
    Interesting... my application is to give my shop a 110v supply out of the 230v european single phase power that runs West Africa. In my case, it'll be coming out of an off grid solar power system. Some of my small power tools are U.S. tools, since tools are so much cheaper here, so I need the 110v to run them.

    Am I understanding correctly that you're proposing a way to wire both 230 in and get 110 out of just the secondary connections, leaving the primary disconnected?

    Here's a pic of the transformer, with the connection diagram:
    Shared album - Jason Atkins - Google Photos

    I can't immediately see a way of configuring the jumpers that would allow such an arrangement.

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    Yes leave the primary disconnected. Put 230v across x1 and x4, connect x2 to x3 and you have 110 volts from both x1 and x4 to x2and3.

    The transformer only has to supply the difference if you have loads wired to both x1 and x4 from x2/3.

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    Connect 240 to X1 and X4. Connect X2 and X3 together. Between X1 and X2,X3 you will get 120 volts. Same with X4 and X2,X3. Each individual secondary winding is rated at 12.5 amps (3000/240). When you use the transformer as an autotransformer, then each secondary winding is 1.5 KVA for a total of 3 KVA.

    Tom

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    Quote Originally Posted by johansen View Post
    Yes leave the primary disconnected. Put 230v across x1 and x4, connect x2 to x3 and you have 110 volts from both x1 and x4 to x2and3.

    The transformer only has to supply the difference if you have loads wired to both x1 and x4 from x2/3.
    Wonderful! The body of knowledge of this forum's members is impressive! Thanks a lot, guys.

    Now, for a backup question... My standard single phase outlet arrangement in the shop is an american 110v outlet alongside a european 230v outlet. In order to power those, can I run a common neutral (presuming the combined loads aren't more than the neutral should carry)? In other words, I'm putting a 230v hot and neutral on x1 and x4. Let's assume x1 is the 230 hot. So can the 230v neutral connected to x4 also serve as the neutral for the 110v circuit, whose hot is x2/x3? It seems that it must be able to do so, since there isn't another neutral to grab from anywhere else. (I realize this whole scenario is out in left field, but my situation is kind of a strange one!)

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    Yes you can do that, the neutral will carry the sum of both 110 and 230v loads.


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